There are many applications for non-contact gages which can measure the size of a gap between a probe and a surface. The general principle involved is that a gas jet can be partially blocked by the presence of an object or a surface. The percentage of blocking is roughly proportional to the distance of the probe from the object or surface. Conventionally, the variation in system pressure or the gas flow caused by the blockage is read as the size of the gap. In most applications the gas to be used will be air. However, other gases do offer substantial advantages in special applications, and the term "gas" as used herein is intended to be generic to all suitable gases.
Many such instruments use a high pressure positive flow of gas, and the gas from the instruments blows into the surroundings. For many applications there is no objection to this flow. However, when exquisitely small measurements must be made on the order of one or two millionths of an inch, too high a flow of gas becomes a problem. Adiabatic expansion at the jet causes errors from local cooling, and large amounts of turbulent gas stimulate dust movement which would ordinarily lie dormant. This is not acceptable in clean rooms because it causes work scrappage. Efforts to avoid particulate introduction include very extensive filtration of gas, and the use of specially prepared and temperature controlled gas, usually from an external location. This is expensive, troublesome, and generally only partly successful.
Very low pressure gas systems, such as disclosed in Wilson U.S. Pat. No. 4,142,401, cause far fewer difficulties and are now being used, for autofocusing of semiconductor cameras which are operating at the leading edge of U.S. technology for sub-micron integrated circuit manufacture. These instruments perform to a very high degree of accuracy. The Wilson patent is incorporated herein by reference in its entirety for its showing of the use of a gas gage capable of measuring extremely small gaps.
However, this invention provides an improvement over all previous systems including the system shown in the Wilson patent by utilizing a basically new concept of employing gas in an "alternating current" or "a.c" fashion instead of in the "direct current" or "d.c." gas flow previously used. This invention moves a tiny puff of gas in one direction, and then sucks it back immediately. This alternating action can take place at from a few times per second, up to many thousands of times per second, depending upon the type of sensor used and the net system data speed resolution required by the user. In this invention, there is no net flow of gas into the surrounding regions, and the risk of contaminating the work pieces is greatly reduced.
The pump for generating this alternating gas flow can be of many types, including magnetic solenoids, piezoelectric motion generators, moving coil loud speaker devices, and the like. They may be made to resonate mechanically to improve output or to reduce power requirements. These devices can be very tiny and can be placed right on the probe conduit being used for the measurement sensing.
The high speed sensors used for "a.c." gas gaging must be rapid enough to respond to the gas pump oscillations. They may be the hot-wire anemometer type described in Wilson patent. They may also be a high speed diaphragm type which can measure dynamic pressure changes. Throughout this specification, the term "dynamic" is used in the sense of causing or sensing properties which are changing rather than static. A dynamic device will not measure a static property.
The configuration of the gaging instrument can vary from simple single conduit sensing jets with one single sided sensor, to dual balanced units using an active jet and a reference jet to cancel out some external interfering effect such as room temperature variation and ambient gas pressure variations from air conditioners and doors opening and closing.
It is significant to this invention that in the electronic circuitry great advantage can be taken in using "synchronous demodulation". This technique can detect coherent and useful data in a background of greatly higher noise and static.
Perhaps the most significant object of this invention is to provide an instrument which does not introduce particulates into the surrounding region.
It is another object of this invention to permit measurements considerably smaller than one millionth of an inch. This is accomplished, resulting in very simple equipment with a reduced sensitivity to air currents, drafts, and stray pressure variations such as are often caused by air conditioners and the opening and closing of doors. This invention reduces measurement errors caused by such events to less than one millionth of an inch.
Still another object of this invention is to eliminate substantial plumbing, valves, and tubings leading to the sensing orifice. The plumbing especially the tubings, is a source of data artifacts whenever the probe is physically scanned across a workpiece surface to create a chart profile. Bending and bumping of hoses can cause gas pumping that creates data artifacts in d.c. gas systems, and special precautions must be taken to minimize them. This invention substantially overcomes this problem.
Yet another object of this invention is to provide an instrument which has only minimal power consumption. In fact it may be less than one watt including the gas supply. Such power is readily supplied by a battery, thereby further decreasing the bulk and complexity of the instrument. A gas supply for this instrument can operate on a power of 0.15 watts. This is an improvement by a factor of one hundred over previous units, and requires almost no warm-up time to reach equilibrium.